The thermal cis-trans isomerization of cis-transoidal polyphenylacetylene (PPA) synthesized with Noyori's catalyst [Rh(CdropCPh)(norbornadiene)(PPh3)(2)] has been investigated under both ambient and inert atmospheres in solution and in bulk. In all cases, an intramolecular cyclization results in cis-trans isomerization, and subsequent chain cleavage produces 1,3,5-triphenylbenzene. This reaction is accelerated by the presence of air, particularly when the reaction takes place in solution. Decreases in the cis content and molecular weight show that the intramolecular cyclization process takes place at 23 degreesC in solution. The mechanism of this reaction is identical to that reported previously for cis-cisoidal and cis-transoidal PPA synthesized with Ziegler-Natta catalysts.